This invention relates to an abnormality diagnosis apparatus for an evaporative emission control system which prevents emission of evaporative fuel generated in a fuel tank from which fuel is supplied to an internal combustion engine, and more particularly to an abnormality diagnosis apparatus for such a system that the emission of evaporative fuel is prevented by maintaining the pressure in a fuel tank at a negative pressure.
An apparatus is disclosed, for example, in Japanese Patent Laid-Open No. Hei 10-281019 wherein an intake pipe of an internal combustion engine and a fuel tank are connected directly to each other by an evaporative fuel passage and the pressure in the fuel tank is maintained at a negative pressure (pressure lower than atmospheric pressure) to prevent emission of evaporative fuel. Also an abnormality diagnosis method is disclosed, for example, in Japanese Patent Laid-Open No. Hei 5-195881 or Japanese Patent Laid-Open No. Hei 9-317572, wherein a canister for temporarily storing evaporative fuel is provided and usually the pressure in a fuel tank is maintained at a pressure around atmospheric pressure, whereas the pressure in the fuel tank is reduced to a negative pressure only when the abnormality diagnosis is performed.
In the conventional abnormality diagnosis method described above, the pressure in the fuel tank is set to a negative pressure after setting the pressure in the fuel tank to atmospheric pressure, and an abnormality is diagnosed based on the change in the pressure in the fuel tank during such a procedure. Therefore, if the conventional abnormality diagnosis method is applied to an evaporative emission control system where the pressure in the fuel tank is normally maintained at a negative pressure to prevent emission of evaporative fuel, then it is necessary to raise the pressure in the fuel tank to atmospheric pressure to diagnose an abnormality. Therefore, after the abnormality diagnosis comes to an end, the pressure in the fuel tank must be reduced to a negative pressure again. Consequently, a problem arises that energy required to control the pressure in the fuel tank to a negative pressure is consumed wastefully.
It is an object of the present invention to provide an abnormality diagnosis apparatus which can diagnose an abnormality such as a failure of a control valve or a leak in a canister used in an evaporative emission control system in which the pressure in a fuel tank is normally maintained at a negative pressure to prevent emission of evaporative fuel, maintaining the pressure in the fuel tank at a negative pressure, to thereby eliminate wasteful consumption of energy.
In order to attain the object described above, according to the present invention, there is provided an abnormality diagnosis apparatus for an evaporative emission control system which includes a fuel tank, a canister for adsorbing evaporative fuel generated in the fuel tank, a charging passage for connecting the canister and the fuel tank, a purge passage for connecting the canister and an intake system of an internal combustion engine, a vent passage for opening the canister to atmospheric air, an evaporative fuel passage for connecting the fuel tank and the intake system, a first control valve interposed in the charging passage for opening and closing the charging passage, a second control valve interposed in the purge passage for opening and closing the purge passage, a third control valve interposed in the vent passage for opening and closing the vent passage, a fourth control valve interposed in the evaporative fuel passage for opening and closing the evaporative fuel passage, and control means for controlling an opening of the fourth control valve so that a pressure in the fuel tank may be maintained at a predetermined pressure lower than atmospheric pressure at least during operation of the internal combustion engine. The abnormality diagnosis apparatus comprises tank pressure detecting means for detecting the pressure in the fuel tank; and abnormality diagnosis means for diagnosing abnormality of the evaporative emission control system based on an output of the tank pressure detecting means; wherein the controlling means outputs a control signal for maintaining the fourth control valve in a closed state while an abnormality diagnosis is performed by the abnormality diagnosis means, and the abnormality diagnosis means performs an abnormality diagnosis by outputting a control signal for changing an open or closed state of at least one of the first, second and third control valves without opening the first and third control valves simultaneously.
With this configuration, the abnormality diagnosis is performed by outputting a control signal for maintaining the fourth control valve, which opens and closes the evaporative fuel passage, in a closed state and outputting another control signal for changing an open or closed state of at least one of the first control valve, which opens and closes the charging passage, the second control valve, which opens and closes the purge passage, and the third control valve, which opens and closes the vent passage, without opening the first and third control valves simultaneously. Therefore, the internal pressure of the fuel tank can be maintained at the negative pressure throughout execution of the abnormality diagnosis, and pressure loss by the abnormality diagnosis can be prevented to eliminate wasteful consumption of energy.
Preferably, the abnormality diagnosis means outputs control signals for closing the first, second and third control valves, outputs a control signal for opening the first control valve, and determines an open failure of the second control valve or a close failure of the first control valve based on a change in the pressure in the fuel tank after outputting the control signal for opening the first control valve.
Preferably, the abnormality diagnosis means determines that an open failure of the second control valve or a close failure of the first control valve has occurred when the amount of change in the pressure in the fuel tank is smaller than or equal to a first predetermined change amount.
Preferably, the abnormality diagnosis means outputs control signals for closing the first, second and third control valves, outputs a control signal for opening the first control valve, outputs a control signal for opening the second control valve, and determines a close failure of the second control valve based on a change in the pressure in the fuel tank after outputting the control signal for opening the second control valve.
Preferably, the abnormality diagnosis means determines that a close failure of the second control valve has occurred when the amount of change in the pressure in the fuel tank is smaller than or equal to a second predetermined change amount.
Preferably, the abnormality diagnosis apparatus further comprises intake air pressure detecting means for detecting an intake air pressure in the intake system. In this apparatus, the abnormality diagnosis means inhibits determination of a failure of the second control valve when an absolute value of a difference between the pressure in the fuel tank and the intake air pressure in the intake system is less than or equal to a predetermined value.
Preferably, the abnormality diagnosis means outputs control signals for maintaining the first, second and third control valves in a closed state in a condition where a pressure in the canister is lower than atmospheric pressure, outputs a control signal for opening the first control valve at a time when a predetermined stabilization period has elapsed, and determines whether or not there exists a leak in the canister based on a change in the pressure in the fuel tank after outputting the control signal for opening the first control valve.
Preferably, the abnormality diagnosis means outputs control signals for maintaining the first, second and third control valves in a closed state after the determination of a close failure of the second control valve, outputs a control signal for opening the first control valve at a time when a predetermined stabilization period has elapsed, and determines whether or not there exists a leak in the canister based on a change in the pressure in the fuel tank after outputting the control signal for opening the first control valve.
Preferably, the abnormality diagnosis means determines that there exists a leak in the canister when the amount of change in the pressure in the fuel tank is greater than or equal to a third predetermined change amount.
Preferably, the abnormality diagnosis means determines that there exists a leak in the canister when the amount of change in the pressure in the fuel tank is greater than or equal to a third predetermined change amount.
Preferably, the abnormality diagnosis means outputs control signals for maintaining the first, second and third control valves in a closed state in a condition where a pressure in the canister is lower than atmospheric pressure, outputs a control signal for opening the third control valve, outputs a control signal for closing the third control valve, outputs a control signal for opening the first control valve, and determines a close failure of the third control valve based on a change in the pressure in the fuel tank after outputting the control signal for opening the first control valve.
Preferably, the abnormality diagnosis means determines that a close failure of the third control means has occurred when the amount of change in the pressure in the fuel tank is smaller than or equal to a fourth predetermined change amount.
The above and other objects, features and advantages of the present invention will become apparent from the following description and the appended claims, taken in conjunction with the accompanying drawings in which like parts or elements denoted by like reference symbols.